In a brief statementon its blog Tuesday , Iain Mulholland, director of VMware's security response center, said that the company was aware of a "single file" from the VMware ESX source code, and associated commentary from software developers, being publicly posted on Pastebin.com. The code and commentary date back to 2003 and 2004, he said.

Mulholland said VMware discovered the file had been publicly posted on Monday. He warned that more files containing similar source could be posted in future.

"The fact that the source code may have been publicly shared does not necessarily mean that there is any increased risk to VMware customers," Mulholland noted. "VMware proactively shares its source code and interfaces with other industry participants to enable the broad virtualization ecosystem today," he said, appearing to imply the leak could have happened elsewhere.

"We take customer security seriously and have engaged internal and external resources, including our VMware Security Response Center, to thoroughly investigate," he added.

Kaspersky Labs' ThreatPost blog reported on Tuesday that a hacker named 'Hardcore Charlie' claimed having downloaded 300MB of VMware source code from a system belonging to the China Electronics Import-Export Corporation (CEIEC), a company that does contracting work for the Chinese military. It is not immediately clear how VMware's source code ended up on CEIEC's computers.

Hardcore Charlie had earlier this month announced he had broken into CEIEC's computers and extracted thousands of documents from the company's servers. The VMware source code that was posted on Pastebin this week appears to have been one of those documents.

"While details are sketchy, this attack once again shows that even the best prepared firms can have risks from consequential third-party access to data out of their control," Mark Bower, a vice president at Voltage Security, said in an emailed statement. "The real pain for the industry in this case is less about counterfeit VMware instances, but the intimate knowledge attackers may now possess of possible vulnerabilities in a critical virtualization tool."

Earlier this year, the security vendor Symantec also suffered a similar source code leak. In Symantec's case, the leaked code involved the Symantec Endpoint Protection 11.0 and Symantec Antivirus 10.2 products, both of which were more than five years old at the time they were posted.

One of the purloined documents described an Application Programming Interface (API) for Symantec's antivirus product. Another listed the complete source code tree file for Norton Antivirus. An Indian hacking group calling itself Lords of Dharmaraja claimed that it had accessed and posted the source code.

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